1. Field of the Invention
The present invention relates to a cushioning material for a polishing pad, used for flattening a semiconductor wafer or a wafer during a circuit forming process, and more particularly, to a cushioning material for a polishing pad, realizing uniform flattening polishing of an entire wafer with high accuracy.
2. Description of the Related Art
A semiconductor wafer is flattened by being chemical-mechanically polished in a method called a chemical mechanical polishing method (hereinafter, abbreviated as CPM method). A polishing device adopting the CPM method employs a polishing pad. The polishing pad includes a surface layer (upper layer) and a lower layer. The surface layer is formed of a hard material (hereinafter, referred to as surface layer material) and the lower layer is formed of a cushioning material (soft material).
Conventionally, for the cushioning material of the polishing pad, a polyurethane foam is widely used. However, the cushioning material made of the polyurethane foam has air permeability, so the cushioning material absorbs slurry water used for polishing to be swollen during polishing process. As a result, “permanent set” is caused partially or entirely in the cushioning material, or elasticity thereof is changed. The polishing pad has a structure in which a surface layer material and the cushioning material are formed integrally with each other. Therefore, advancement in swelling deformation of the cushioning material affects the surface layer material, thereby disabling uniform polishing with accuracy. Thus, there arises a problem of replacement frequency for the polishing pad becoming higher.
In this case, in a conventional technique, in order to solve the above-mentioned problem, there are proposed a material including a flexible film made of polyethylene or the like, serving as a waterproof material layer interposed between the surface layer material and the cushioning material (see, for example, JP 11-156701 A (claim 1, 0016)), and a cushioning material including a polyurethane foam with small variation in thickness obtained by using a silicone foam stabilizing material having a functional group, thereby reducing a water vapor permeability to adjust foaming density and hardness, the polyurethane foam having adhesive layers on both surfaces thereof (see, for example, JP 2004-253764 A (claim 1, 0008)).
However, with the structure in which the waterproof material layer is interposed between the surface layer material and the cushioning material, there is a risk of work-hour being increased for sticking the waterproof material (film) and, because the waterproof material is the film, a wrinkle being caused at a time of sticking the film. There is a problem in that, due to the wrinkle on the film, uniformity of the thickness of the pad is impaired, costs increase, or the like. Further, even when the waterproof material layer is provided between the surface layer material and the cushioning material, intrusion of water from a side surface of the cushioning material cannot be prevented, so the structure is imperfect.
Further, the structure in which the adhesive layers are provided to the both surfaces is imperfect for preventing intrusion of water in a state where, even if the cushioning material itself has low moisture absorption, the cushioning material repeats compression and restoration.